Can Blender build proxy files with transparency?
I've found this solution:

Blender looks for jpeg files, but actually you can use PNG files with .jpg extension.

This is my workflow:

1. Create resized copies of the render images:

25% > 1920 x 1080 -> 480 x 270

2. Change all the extensions from .png to .jpg

Assuming that the render path was:

/User/render/def/barca/b.png


the Proxy Directory in the VSE Panel was

/User/render/proxy/


I've used ffmpeg to create a scaled copy of the render files.

Open terminal (I'm on Mac) and type:

cd /User/render/

ffmpeg -i def/barca/b-%04d.png -vf scale=480:-1 proxy/images/25/b-%04d.png_proxy.png


than rename all the files

cd proxy/images/25/
for old in *.png; do mv $old `basename $old .png`.jpg; done


(thanks: https://stackoverflow.com/questions/14887275/changing-file-extensions-for-all-files-in-a-directory-osx)

Here's a rough script that sets all the strips

import bpy
C = bpy.context
C.scene.sequence_editor.proxy_storage = 'PROJECT'

C.scene.sequence_editor.proxy_dir = '/path/for/proxy/'

seq = C.scene.sequence_editor.sequences_all
for s in seq:
    s.use_proxy = True
    s.proxy.use_overwrite = False

    # I ONLY NEED PROXY 25%
    s.proxy.build_25 = True